Opal is a unique mineraloid, distinct from crystalline gemstones because it is a hydrated amorphous form of silica, lacking a defined crystal structure. Its composition includes silica and a variable amount of water, typically ranging between six and ten percent of its weight. The allure of the stone lies in its characteristic display of shifting spectral hues known as “play-of-color.” This guide provides practical instructions for beginners seeking to locate and identify raw opal.
Understanding Opal Formation
The presence of opal is a result of specific geological conditions combining over long periods, often in arid or semi-arid environments. Opal forms when silica-rich water percolates down through the earth, dissolving silica from host rocks like sandstone. This silica-infused solution collects in cracks, faults, and voids, which may include spaces left by decomposed fossils or ironstone nodules.
The formation process relies on the evaporation of the water, which leaves behind a deposit of amorphous silica gel. Repeated cycles of wetting and drying, often linked to a fluctuating water table, concentrate this silica, slowly hardening it into an opal deposit. This process often occurs within deeply weathered Cretaceous sedimentary rocks, where claystone layers, sometimes called “opal dirt,” act as the host material and serve as a key indicator of potential opal-bearing ground.
Essential Tools and Preparation
Successful prospecting begins with thorough preparation and the right equipment. Basic safety gear includes a wide-brimmed hat, sturdy leather gloves, protective footwear, and a comprehensive first-aid kit. Because opal fields are often in hot, dry environments, carrying a sufficient supply of water for both drinking and field testing finds is non-negotiable.
For the actual search, a beginner’s toolkit should include a small pickaxe, a shovel for moving overburden, and a trowel for precise digging. A mesh sieve or screen is important for sifting material to separate small stones from fine dirt. Before setting out, research local regulations to identify public fossicking areas and understand permitting or claim laws, as digging on an active mining claim without permission is prohibited.
Prospecting Techniques
The most accessible method for beginners is “noodling,” which involves searching through the heaps of waste material, or mullock, left behind by commercial miners. Historically, miners often missed small pieces or lower-grade opal, and these tailings piles can still yield valuable finds. Use a small hand shovel to scrape away the top layer of the mullock heap, then carefully look for any exposed color or unusual rocks.
Another technique is “specking,” which means walking slowly over the ground, particularly after rainfall, looking for pieces of “float” or opal chips lying on the surface. The best time for specking is often in the morning, with the sun low behind you, as this angle of light helps catch the brief flash of color from any exposed opal. For more active prospecting, small-scale trenching can be undertaken in designated fossicking areas to expose new ground. Wetting the potential opal-bearing material will remove dust and temporarily enhance any color, making identification easier.
Identifying and Handling Raw Opal
The primary feature distinguishing precious opal from common rock is the “play-of-color,” an internal phenomenon where light is diffracted by the uniform structure of microscopic silica spheres. When you rotate a piece of rough stone, precious opal will exhibit distinct, shifting flashes of spectral color. Common opal, sometimes called “potch,” lacks this uniform sphere arrangement, appearing instead as a milky, matte, or earthy-toned stone without the characteristic fire.
Raw opal often exhibits a slightly waxy or greasy luster and may feel lighter than a comparably sized piece of quartz due to its lower specific gravity. Handle finds with care, as opal contains water and is susceptible to damage from rapid environmental changes. To prevent “crazing,” a network of fine cracks caused by rapid dehydration, newly found raw opal should be stored in a soft cloth or a sealed container to protect it from extreme heat, direct sunlight, or sudden temperature shifts.